1. Field of the Invention
The present invention relates to metal oxide semiconductor field effect transistor (MOSFET) amplifiers with cascode outputs, and in particular, MOSFET amplifiers with cascode outputs biased to maintain operation of their driver transistors in states of saturation.
2. Description of the Related Art
Many amplifier circuits using MOSFETs (P-type or N-type, or both) are increasingly relying upon the use of cascode output devices for maintaining or increasing the amplifier output impedance. This has become increasingly important as transistor sizes, particularly MOSFET channel lengths, become smaller with increased integrated circuit densities. This is true for current mirror circuits (e.g., as discussed in more detail in U.S. Pat. No. 4,550,284, the disclosure of which is incorporated herein by reference) as well as signal amplifiers.
One type of signal amplifier relying upon cascode output transistors is that in which the signal to be amplified or buffered is an AC signal centered about a DC baseline, or common mode, voltage. Such an amplifier, whether differential or single-ended, often relies upon the use of a current mirror circuit for providing the amplifier biasing current to the transistor or transistors responsible for amplifying the incoming signal. Following such transistor or transistors, is the cascode output circuitry responsible for maintaining or increasing the output impedance.
Two difficulties are often encountered in biasing such circuitry. One difficulty involves the biasing of the current mirror providing the amplifier biasing current. Depending upon the magnitude and stability of the DC power supply voltage, the biasing of the current mirror circuitry can vary, thereby causing the amplifier biasing current to vary as well. A second difficulty concerns the biasing of the cascode output circuitry. Such biasing is usually done by applying a fixed voltage relative to the power supply voltage. However, while such bias voltage may remain constant, the baseline, or common mode, voltage associated with the input signal may vary, thereby preventing the input transistors from operating in true states of saturation, particularly over PVT (i.e., variations in device fabrication Processing, power supply Voltage and operating Temperature).
In accordance with the presently claimed invention, a metal oxide semiconductor field effect transistor (MOSFET) amplifier includes dynamically biased cascode output circuitry in which the biasing of the cascode output circuitry dynamically tracks one or more other internal amplifier bias voltages such that operation of each transistor in the input signal circuitry is maintained in a state of saturation.
In accordance with one embodiment of the presently claimed invention, a metal oxide semiconductor field effect transistor (MOSFET) amplifier with dynamically biased cascode output circuitry includes power supply terminals, telescopic cascode amplifier circuitry, voltage replication circuitry and voltage translation circuitry. The power supply terminals convey first and second voltages defining a power supply voltage. The telescopic cascode amplifier circuitry, coupled between the power supply terminals, responds to reception of a current source bias voltage intermediate the first and second voltages, an input signal centered about an input baseline voltage intermediate the first and current source bias voltages, and at least one cascode bias voltage intermediate the first and input baseline voltages by providing a first internal bias voltage intermediate the current source bias and input baseline voltages, a second internal bias voltage intermediate the input baseline and at least one cascode bias voltages, and an output signal corresponding to the input signal and centered about an output baseline voltage intermediate the first and cascode bias voltages. The voltage replication circuitry, coupled to the telescopic cascode amplifier circuitry, responds to reception of the input baseline voltage and a first bias current by providing the current source bias voltage and a first replica bias voltage substantially equal to the first internal bias voltage. The voltage translation circuitry, coupled to the voltage replication circuitry and the telescopic cascode amplifier circuitry, responds to reception of the current source bias voltage, the first replica bias voltage and a second bias current related to the first bias current by providing a first one of the at least one cascode bias voltage.
In accordance with another embodiment of the presently claimed invention, such MOSFET amplifier with dynamically biased cascode output circuitry further includes additional voltage replication circuitry and additional voltage translation circuitry. The additional voltage replication circuitry, coupled to the first voltage translation circuitry, responds to reception of the first one of the at least one cascode bias voltage and a third bias current related to the first bias current by providing a current mirror bias voltage and a second replica bias voltage substantially equal to the second internal bias voltage. The additional voltage translation circuitry, coupled to the additional voltage replication circuitry and the telescopic cascode amplifier circuitry, responds to reception of the current mirror bias voltage, the second replica bias voltage and a fourth bias current related to the first bias current by providing a second one of the at least one cascode bias voltage.